Oversizing PFC caps

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ptonsparky

ptonsparky

Tom
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
EC - retired
Working at a small industrial that is a sister to another customer just a few miles away. Manger pretty much says do what you need to do. So I took it upon myself to check out his Power Factor penalties. Close to $5k per year. The sister plant received $634 in credits last year after we made corrections there.

I know the best way is to correct each motor. Cheaper is to correct in one spot with one or two larger ones. We are adding a few motors & moving a couple others. What would be the downside to oversizing these caps? Shoot for 1 instead of .92. I know the heater sizes would need to be reduced. But what else? Reason I ask this is they have limited ampacity of spare disconnets just to serve PFC caps. Seems like oversizing on the largest of the motors may help.
 
Depends on the cost. As you approach unity PF, it costs more to correct a little more. Also (depends on the utility), you may go past unity and the overcorrection will increase the kVA and the customer's cost may be increased.

In my opinion, it is best to do the correction close to the service with automatic equipment. :smile:
 
ptonsparky said:
What would be the downside to oversizing these caps? Shoot for 1 instead of .92. I know the heater sizes would need to be reduced. But what else? Reason I ask this is they have limited ampacity of spare disconnets just to serve PFC caps. Seems like oversizing on the largest of the motors may help.
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Make the decision on the return on the investment. There is nothing wrong with correcting to a PF higher the 0.92. Suggest that you do not over correct. 0.98 is a nice figure. I believe that based on savings, you can make a case for improving the PF to a higher value. You may want to install a bank of cap at the main and additional caps on the motors. Be sure that there are no VFD's in place. If so, take a step back.
 
ptonsparky said:
Working at a small industrial that is a sister to another customer just a few miles away. Manger pretty much says do what you need to do. So I took it upon myself to check out his Power Factor penalties. Close to $5k per year. The sister plant received $634 in credits last year after we made corrections there.

I know the best way is to correct each motor. Cheaper is to correct in one spot with one or two larger ones. We are adding a few motors & moving a couple others. What would be the downside to oversizing these caps? Shoot for 1 instead of .92. I know the heater sizes would need to be reduced. But what else? Reason I ask this is they have limited ampacity of spare disconnets just to serve PFC caps. Seems like oversizing on the largest of the motors may help.
Click to expand...
The generally accepted value to correct t is .95. As mentioned above, the cost to get more than that exceeds the benefits; the law of diminishing returns.

Doing what is called "At Load" correction is really the best way, because by only having the capacitors come on with each "offender" means you will never over-correct. And by the way, some PoCos will penalize you for over-unity pf the same as for having low pf!

But I don't understand this comment: "Reason I ask this is they have limited ampacity of spare disconnects just to serve PFC caps." Sounds to me as though you might be misunderstanding something here, but rather than my second guessing what you were trying to say, please expand that thought a little.
 
Having the capactiors at the load also reduces the in-plant losses on the wiring between the service and the motor loads.

These may be small compared to the PF penality the POCO imposes, but it is something to consider.


Steve
 
In order to put in one or two capacitors to serve plant wide correction I need a fused disconnect(s) of adequate size. We have only one 225 amp bucket left for use plus a 30s. Most likely a 100KVAR could be connected full time. I do not want to overcorrect because it is a waste of money. After I get an idea from the PFC sales reps as to what they suggest I will try to make up the difference at selected motors with slightly oversized caps. My goal is .91, from the current .82.

There are only two small VFDs and one 250 hp softstart to contend with. Electrically I will be over 150 ft from the soft start and the the smaller drive. The other about 75 feet. Distance helps, but is no guarantee, from what I understand.
 
charlie said:
You have just gone past my level of expertise. One of our engineers will need to answer this question. :smile:
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Computer modeling(at a $cost) will determine if their is a possibility of system resonance.Usually if the installed kvar is quite low compared to the system short circuit values and it is predominanly ac loads(no large dc drives etc.)there is not a problem.:smile:
 
ghostbuster said:
Computer modeling(at a $cost) will determine if their is a possibility of system resonance.Usually if the installed kvar is quite low compared to the system short circuit values and it is predominanly ac loads(no large dc drives etc.)there is not a problem.:smile:
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ghost -
Could you pin down the relationships some more?

For example, I'm translating "installed kvar is low compared to system short circuit values" to "capacitive current is low compared to system available short circuit current". Is that correct?

I'm also not clear on the "predominanly ac loads (no large dc drives). I'm seeing a large AC drive front end as looking just like a large DC drive front end. Is there a difference? Or are you saying, "No high harmonic content loads?"

Would you have any references you could suggest?

cf
 
Cold Fusion said:
ghost -
Could you pin down the relationships some more?

For example, I'm translating "installed kvar is low compared to system short circuit values" to "capacitive current is low compared to system available short circuit current". Is that correct?

I'm also not clear on the "predominanly ac loads (no large dc drives). I'm seeing a large AC drive front end as looking just like a large DC drive front end. Is there a difference? Or are you saying, "No high harmonic content loads?"

Would you have any references you could suggest?

cf
Click to expand...

CF

As a primer you could start with this book.

http://www.amazon.com/Electrical-Power-Systems-Quality-Santoso/dp/007138622X


Then continue on into the I.E.E.E applications transactions.



Answer 2

Usually if you need to install 60-100 kvar of correction and the service size is 2000-3000 amps(40,000-60,000 amps fault level),there should not be a problem.
I had one customer that had almost 95% dc drives (harmonic producer).This client hit resonance at the 23rd harmonic with 60 kvar.The current into the cap was almost double.Fuses blew at the same the cap blew up(in approx. 10 seconds).This is the odd case however.:smile:
 
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